Soil Accumulation of Nitrogen and Phosphorus Following Annual Fertilization of Loblolly Pine and Sweetgum on Sandy Sites

Abstract

A key question that arises following fertilization of forest plantations in the southeastern United States is whether fertilization has a long-term effect on nutrient availability and long-term site quality. Site quality could improve if mineral soil N and P pools are increased but is less likely if only forest floor pools are increased. Rapid forest floor decomposition and nutrient release following harvest likely lead to only rotation-length increases in nutrient availability. To examine the impact of repeated applications of N and P, forest floor and mineral soil N and P pools were measured at a 24-yr-old loblolly pine (Pinus taeda L.) plantation in North Carolina (Southeast Tree Research and Education Site [SETRES]) and 13-yr-old loblolly pine and sweetgum (Liquidambar styraciflua L.) plantations in Georgia (Mt. Pleasant) receiving annual incremental fertilizer applications totaling >1000 kg N ha−1 and >160 kg P ha−1. Mineral soil N pools varied only slightly. In contrast, fertilization increased forest floor N pools by 300 kg N ha−1 at SETRES. Long-term impacts on site N availability and site quality following these large N applications are unlikely. The forest floor P pool at SETRES was increased by 19.8 kg P ha−1. Fertilization also increased the mineral soil extractable P pool at SETRES to a depth of 1.5 m by 56 kg P ha−1 but not at Mt. Pleasant where P fertilization during the previous rotation may have already increased this pool. Observed increases in mineral soil P from fertilization indicated a potential for increased long-term site quality in terms of higher P availability for future rotations.

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